The Portal to Ooo
by PureBlossom
Summary: Set in the modern day, Pen, Fiona and Finn are the guardians of a secret portal leading to the land of Ooo. It opens every full moon, and their responsible for making sure nothing gets out into their world. But this time something gets into Ooo, and Pen, Fiona and Finn, with help from a few friends, have to try and save Ooo and their world.
1. Chapter 1

**I Don't Own Adventure Time. Sorry. **

Chapter One: Full Moon Tomorrow

"Hey, Pen, can Marcy come over? I promise we'll be good."

Pen sighed, rubbing his forehead. "Finn, Marceline's my age, why don't you get friend's your own age?"

"Aw, phooey, does that mean she can't come over?" Ten-year-old Finn muttered, pouting. Pen, his eighteen-year-old brother, looked over his shoulder.

The blonde brothers were nearly identical. In fact, everyone in their family was. Pen, Fionna and Finn, all blonde hair and pale blue eyes.

"Fine, Finn, but…" Pen trailed off. Really, what was his problem with it? So what is Finn wanted to hang out with people eight years older than him.

"Yay!" Finn exclaimed, jumping away and running off into their house. Pen sighed again, and put his book down. These days, between taking care of Finn and Fionna and work he never got a moment for himself. Better tidy the house for the company. No doubt Fionna would be bringing someone back, too.

As if on cue, Fionna opened the door and dropped her bag at the entrance.

"Tell me again why Finn got to skip school?" She asked, folding her arms.

"He didn't skip school. He got suspended for calling the teacher an 'Idiot who couldn't do math if he had a calculator and the answer sheet', remember?" Pen answered, watching Fionna dropped down onto the couch.

"Right." She muttered.

"Hey, Finn's dragging Marceline over, anybody coming over on your end?" He asked, brushing a pile of crumbs of the counter. He didn't stop to think about what they came from.

"Yeah, I got Bubblegum coming over to work on a science project." Fionna said, turning to look at him. "Is it okay is she stays the night?"

Pen looked up, concern in his eyes. "No." He said simply. Fiona frowned.

"Why- Oh… Already?" Fionna asked, concerned.

"Yeah, We've got to be ready tomorrow. I'm gonna call your teachers to tell them you won't be at school tomorrow." Pen told her, going back to his cleaning.

"I can't believe it's the full moon already." Fionna said, taking a deep breathe and leaning against the back of the couch.

"Yep, and I'll be a cockatrice before we let any of them near the portal." Pen added.

"You'd think having an inter-dimensional portal in our basement would make us cool. And popular." Fionna muttered. "Instead we're two steps from being social outcasts."

"Hey, mom and dad-" Pen cut himself off, looking down. Fionna looked up.

"Yeah?" She asked. Pen shook his head. He was the only one old enough to actually have an emotional bond with their parents.

"They protected this portal with their lives. It's our job to keep that going. Social outcasts or not." Pen said firmly, watching Fionna out of the corner of his eye as he turned to jump a load of something that backfired from Finn's 'lab'.

"Fine." Fionna sighed, standing up.

Pen walked over to her, wrapping his arm around her. "How about I make you that soup you like for dinner?" He offered. Fionna brighten up.

"Okay!" She exclaimed, forgetting her former disappointment. Pen smiled.

"Go remind Finn about tomorrow, I'll get dinner started." He told her. Fionna smiled, bouncing up the stairs. She knocked twice on the door that said 'FINN' in big, blue letters.

"Yeah?" A voice called from inside.

"It's me, I need to talk to you." Fionna called. She actually heard Finn sigh dramatically, before swinging open the door. Fionna was only four years older than him, but it made quite the height difference.

"Tomorrows the full moon." She said plainly. She was less reserved about it than Pen was, and she knew Finn was even less so.

"Aw! Yeah!" Finn exclaimed. "I forgot that was coming up! I've got to pack my sword, and my hat and my-"

"Finn, calm down, it's one day." Fionna pointed out.

"Yeah? So?" Finn said, in that childish sarcastic way.

"So don't get over your head. You know we should take this seriously. " Fionna scolded.

"Yeah, but I mean, the Land of Ooo!" Finn exclaimed. "We'll get to see Jake, and Cake, and maybe LSP will drop by!" He rambled on. Fionna couldn't help but smile. Her brother was contagious.

"I know, I know… But it's just standing by the entrance and staying up all night." Fionna said. Finn didn't seem to care.

"So? It's still inter-dimensional travel." He said. "Now go! I've got to prepare!" He said, trying to push her from the doorway.

Fionna frowned. She wasn't the lightest of girls, with a tiny bit of a stomach. In short, too heavy for ten-year-old Finn.

"Don't forget, you've gotten Marcy coming over, so keep everything hidden." Fionna warned as she stumbled backwards. Finn slammed the door, and Fionna sighed. She better get her science project ready- Bubblegum wasn't one for disorder.

**Hey? What did you think? It's the first thing I've posted on this site, and my first try writing AT. It's a bit shorter than I'd planned... Next will be longer! Please Comment and review! Also, I'd like to know if you want to see any specific pairings...**


	2. Chapter 2

**-Chapter Two: Marcy and Bubblegum-**

_Knock Knock Knock_

Three knocks… "Fionna!" Pen called, waiting for his sister's footsteps. Almost immediately, Fionna came bursting down the stairs, swinging around the corner and swung open the door.

Bubblegum stepped in, shaking of her umbrella and stepping inside. Pen briefly noted that it was raining.

Bubblegum was tall for a fourteen year old, with long hair she dyed bright pink, and pink almost everything else. She wore a skirt and a cardigan, both in pastel pink.

"Hey Fionna, hi Pen!" SHe said cheerfully, stepping out of her shoes. Also pink.

"Hey Bonnabel." Pen called back. Bubblegum smiled.

"Please, call me Bubblegum." She told him. Pen shrugged.

"Come on, I've got everything set upstairs." Fionna said, leading Bubblegum up the stairs. Pen leaned back in his chair, picking up his book again. Barely a minute passed before another knock at the door. This time there were too many too fast to count.

"Finn!" Pen called. FInn came literally jumping down the stairs, and swung open the door, still smiling.

Marceline stepped in, and had an entirely different aura than Bubblegum. Grey tank top, blue tight jeans, red knee-high boots, and what Pen had to assume was a guitar case slung over her shoulder. Pen tensed.

"What's this what's going on?" He said, eyeing the instrument carefully.

"Relax Pen, I'm just showing your brother a few chords, that's all." Marceline explained, smiling down at Finn. Pen narrowed his eyes. Finn and loud instruments. He'd have to proceed carefully.

"FIne, right." Pren muttered, shaking his head and sitting back down.

Finn grabbed her arm, dragging her across their entrance and up the stairs, at the same time Fionna and Bubblegum came down.

Finn jumped back, blushing. He'd had a massive crush on Bubblegum for the longest time. It was kinda cute.

"Oh, hey Finn, how's my little knight doing?" Bubblegum asked sweetly, Finn smiled.

"Oh, I'm good!" He exclaimed. Bubblegum looked up, catching sight of Marceline. Pen shivered. How had he forgotten he'd vowed to never have both of them here at the same time?

"Marceline." Bubblegum said.

"Bonnabel." Marceline said back in a sweet tone.

Awkward staring issued.

"Well," Fionna said finally, stretching out the word. "Me and Bubblegum were just going to ask if we could get a drink."

"That's fine, Fionna." Pen called up.

Fionna nodded, and awkwardly made her way around Finn and Marceline. Personally, she liked Marceline and her brother, Marshal. But Bubblegum seemed to have a very large problem with the family, and she didn't want to pry.

She disappeared with her friend into the kitchen, and Finn continued dragging Marceline upstairs.

After a while, Bubblegum and Fiona disappeared and Pen was able to pick up his book again.

He could faintly hear the sounds of Marceline's bass, but was able to ignore it.

The clock ticked on, and eventually Pen fell asleep in his chair. By the time he woke up, it was nearly eleven.

His eyes found the clock and his heart stopped. He had to get everyone out.

"Finn! Fionna!" He called, trying to make his voice sound even. The two siblings and their friends came down, waiting to see what he wanted.

"Hey, yeah, nice seeing you all but I've gotta-thing-I'm doing you've all got to leave." Pen said quickly. Without wasting a heartbeat he reached over and grabbed Bubblegum's arm, pulling her the rest of the way down the stairs, opened the door and pushed her out.

"Hey! Pen! Me and Fionna weren't done!" Bubblegum protested, but Pen ignored her.

He grabbed Marceline, pushing her out the door.

"Hey!" She barely had time to protest before he slammed the door shut.

Fionna and Finn were staring wide-eyed at their older brother, then he slowly turned to face them.

"Fionna, Finn, grab your hats, swords, material's you'll need, we have to leave _now." _He ordered.

Fionna and Finn didn't reply. Instead, they both spun around and disappeared upstairs.

**Hey! Please Comment and Review, (Same thing I'm pretty sure) and let me know what you think! Your opinion actually does matter to me, so...**


	3. Chapter 3

**-Chapter Three: Portal Open-**

Fionna and Finn got dressed quickly, pulling on bunny hat and bear hat respectively. Both strapped on swords. Fionna's a pretty red blade she'd gotten a few trips ago, and Finn's their father's own sword.

They raced downstairs, surprised to find Pen already waiting, cat hat and blue blade ready.  
They went downstairs without a word. Pen took his necklace off, a simple brown rthread with two keys. He unlocked the door, letting them through before closing it behind them. The room looked like a typical den, red couches, small coffee table, and an old wooden door at one end. A ginger and white cat sat on the end of the sofa, and perked up when she saw them coming.

"Aw, hey Cake. Did you miss us?" Fionna asked, scratching the cat's head. She purred, and watched eagerly as Pen took the second key, unlocking the wooden door.

He swung it open, and on the other side was nothing but fields and open sky.

"Is the orb charged?" He called, the ginger cat stood up on two legs, nodding.

"All set and ready, Pen!" She replied, jumping off the couch and racing through the door and into the fields. Finn raced after her with Fionna right behind.

A ginger dog ran to greet them, the four beings smiling like old friends should.

Pen took one last, long look at the basement before closing the door behind him.

"Hey! Hey! Marcy!" Bubblegum called, chasing after the older girl. Marceline sighed, running a hand through her hair.

"What do you want, Bonnibel." She said through pursed lips.

"I left my notes back at their house, I was wondering if you'd come back to get them with me." She asked.

"No. Get them yourself." Marceline snapped.

"Please Marcy! Pen likes you way more than he likes me, and he pushed us out in such a hurry, I don't want him mad at me!" Bubblegum begged. Marceline rolled her eyes.

"So get them after school tomorrow." Marceline offered, her patience wearing thin.

"But the test is tomorrow, if I don't get them I won't be able to study, and if I can't study I can't guarantee I'll ace the test!" Bubblegum complained, trying to think of how to convince her to go back with her.

"No." Marceline said simply. Bubblegum made a fake pouting face.

"Please…"

"No."

"Please…."

"No."

"Please…."

"If I say yes will you _shut up?"_ Marceline asked, sick of the conversation. Bubblegum smiled.

"Yay! It's almost midnight we should hurry!" She grabbed the older girl's hand, dragging her back towards the house.

"Who's that?" Bubblegum stammered, pulling Marceline to a halt. A dark, shadowy figure was opening the door to the boy's house.

"Pen or Fionna, taking out the trash?" Marceline asked, continuing to walk up the street. Bubblegum seemed satisfied, following the dark haired girl.

They knocked twice, and got no reply. Knocked again, nothing. Marceline knocked one last time, and noticed the door was already open. Nervously, she opened the door the whole way, stepping inside the house. Somehow, with it all dead and quiet, it seemed malicing.

"Hello? Fionna?" Bubblegum called, stepping inside.

"Fionna's not here." A gruff, fairly fake voice called.

"Who's that?" Marceline said defensively.

"Her brother… Ah… Pin."

"Pin." Marceline echoed. That's like a weird mixture of Pen and Finn.

"Yeah…" The voice came back.

All of a sudden, a fairly bright green-blue glow lit up the room, forcing the girls to look away. Footsteps echoed, and the light faded.

"After him!" Bubblegum cried, and both girls took off, following the figure down stairs and threw what looked like a door that had been broken open.

They burst through it, into a den of somesort. The figure and the glow could be seen disappearing through another door into a… a…

Field? With Finn, wielding a sword?

Both girls shook their heads before chasing after it, swinging open the door and stumbling through. The door slammed shut behind them, and they gasped.

What… How…

Where were they?

Pen, Fionna and Finn stood, looking down at them with a mixture of annoyance, fear, anger, worry, and downright terror. Fionna and Pen turned, watching the figure run into the distance.

"Did he-" Fionna began, but Pen cut her off.

"Well. This actually got worse."

**Please tell me what you think! I want to know what you want to happen, what you think of the characters, everything! I value your opinion! Thanks!**


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